The present invention relates to optical imaging systems including a polarizing beam splitter (PBS). More specifically, the present invention relates to an optical imaging system including a reflective imager and a Cartesian wide-angle polarizing beam splitter (xe2x80x9cPBSxe2x80x9d) having a fixed polarization axis. The optical imaging system of the present invention is capable for use with xe2x80x9cfastxe2x80x9d (low f-number) optical beams while providing a high contrast ratio. The term optical imaging system is meant to include front and rear projection systems, projection displays, head-mounted displays, virtual viewers, head up displays, optical computing, optical correlation and other similar optical viewing and display systems.
Optical imaging systems may include a transmissive or a reflective imager or light valve. Traditional transmissive light valves allow certain portions of a light beam to pass through the light valve to form an image. By their very function, transmissive light valves are translucent and allow light to pass through them. Reflective light valves, in turn, only reflect selected portions of the input beam to form an image. Reflective light valves provide important advantages, as controlling circuitry may be placed below the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. New potentially inexpensive and compact liquid color display (LCD) projector configurations may become possible by the use of reflective LC microdisplays.
For projection systems based on reflective LCD imagers, a folded light path wherein the illuminating beam and projected image share the same physical space between a polarizing beam splitter and the imager offers a desirable compact arrangement. The present invention analyzes and recognizes a xe2x80x9cdepolarization cascadexe2x80x9d problem that limits the f/# of the illumination optics of traditional optical imaging systems using a PBS based on discrimination between p and s polarization states. Most reflective LCD imagers are polarization rotating; that is, polarized light is either transmitted with its polarization state substantially unmodified for the darkest state, or with a degree of polarization rotation imparted to provide a desired gray scale. A 90xc2x0 rotation provides the brightest state in these systems. Accordingly, a polarized beam of light generally is used as the input beam for reflective LCD imagers. Use of a polarizing beam splitter (PBS) offers attractive design alternatives for both polarizing the input beam and folding the light path.
A PBS is an optical component that splits incident light rays into a first polarization component and a second polarization component. Traditional PBS""s function based on the plane of incidence of the light, that is, a plane defined by the incident light ray and a normal to the polarizing surface. The plane of incidence also is referred to as the reflection plane, defined by the reflected light ray and a normal to the reflecting surface.
Based on the operation of traditional polarizers, light has been described as having two polarization components, a p and a s-component. The p-component corresponds to light polarized in the plane of incidence. The s-component corresponds to light polarized perpendicular to the plane of incidence.
To achieve the maximum possible efficiency in an optical imaging system, a low f/# system is desirable (see, F. E. Doany et al., Projection display throughput; Efficiency of optical transmission and light-source collection, IBM J. Res. Develop. V42, May/July 1998, pp. 387-398). The f/# measures the light gathering ability of an optical lens and is defined as:
f/#=f(focal length)÷D(diameter or clear aperture of the lens)
The f/# (or F) measures the size of the cone of light that may be used to illuminate an optical element. The lower the f/#, the faster the lens and the larger the cone of light that may be used with that optical element. A larger cone of light generally translates to higher light throughput. Accordingly, a faster (lower f/#) illumination system requires a PBS able to accept light rays having a wider range of incident angles.
The maximum incident angle xcex8max (the outer rays of the cone of light) may be mathematically derived from the f/#, F:
xcex8max=tanxe2x88x921((2F)xe2x88x921)
Traditional folded light path optical imaging systems have employed an optical element know as a MacNeille polarizer. MacNeille polarizers take advantage of the fact that an angle exists, called Brewster""s angle, at which no p-polarized light is reflected from an interface between two media of differing index. Brewster""s angle is given by:
xcex8B=tanxe2x88x921(n1/n0),
where n0 is the index of one medium, and n1 is the index of the other. When the angle of incidence of an incident light ray reaches the Brewster angle, the reflected beam portion is polarized in the plane perpendicular to the plane of incidence. The transmitted beam portion becomes preferentially (but not completely) polarized in the plane parallel to the plane of incidence. In order to achieve efficient reflection of s-polarized light, a MacNeille polarizer is constructed from multiple layers of thin films of materials meeting the Brewster angle condition for the desired angle. The film thicknesses are chosen such that the film layer pairs form a quarter wave stack.
There is an advantage to this construction in that the Brewster angle condition is not dependent on wavelength (except for dispersion in the materials). However, MacNeille polarizers have difficulty achieving wide-angle performance due to the fact that the Brewster angle condition for a pair of materials is strictly met at only one angle of incidence. As the angle of incidence deviates from this angle a spectrally non-uniform leak develops. This leak becomes especially severe as the angle of incidence on the film stack becomes more normal than the Brewster""s angle. As will be explained below, there are also contrast disadvantages for a folded light path projector associated with the use of p and s-polarization, referenced to the plane of reflection for each ray.
Typically, MacNeille PBS""s are contained in glass cubes, wherein a PBS thin-film stack is applied along a diagonal plane of the cube. By suitably selecting the index of the glass in the cube, the PBS may be constructed so that light incident normal to the face of the cube is incident at the Brewster angle of the PBS. However, the use of cubes gives rise to certain disadvantages, principally associated with the generation of thermal stress-induced birefringence that degrades the polarization performance of the component. Even expensive pre-annealed cubes may suffer from this difficulty. Also cubes add significant weight to a compact system.
MacNeille-type PBSs reportedly have been developed capable of discrimination between s- and p-polarized light at f/#""s as low as f/2.5, while providing extinction levels in excess of 100:1 between incident beams of pure s or pure p polarization. Unfortunately, as explained below, when MacNeille-type PBSs are used in a folded light path with reflective imagers, the contrast is degraded due to depolarization of rays of light having a reflection plane rotated relative to the reflection plane of the principal ray. As used below, the term xe2x80x9cdepolarizationxe2x80x9d is meant to describe the deviation of the polarization state of a light ray from that of the principal light ray. As light in a projection system generally is projected as a cone, most of the rays of light are not perfectly parallel to the principal light ray. The depolarization increases as the f/# decreases, and is magnified in subsequent reflections from color selective films. This xe2x80x9cdepolarization cascadexe2x80x9d has been calculated by some optical imaging system designers to effectively limit the f/# of MacNeille PBS based projectors to about 3.3, thereby limiting the light throughput efficiency of these systems. See, A. E. Rosenbluth et al., Contrast properties of reflective liquid crystal light valves in projection displays, IBM J. Res. Develop. V42, May/July 1998, pp. 359-386, (hereinafter xe2x80x9cRosenbluth Contrast Propertiesxe2x80x9d) relevant portions of which are hereby incorporated by reference.
Recently, Minnesota Mining and Manufacturing has developed a novel type of birefringent polymeric multi-layer polarizing film (xe2x80x9c3M advanced filmxe2x80x9d). Co-assigned and co-pending parent application 49837USA6E Beam Splitter, describes the use of a such a film as a polarizing beam splitter. European Patent Application EP 0 837 351 A2 attempts to utilize 3M dual brightness enhancing film (DBEF), an early 3M multi-layer film material, in a projection display apparatus having a xe2x80x9cwide anglexe2x80x9d reflecting polarizer. Such reference refers to p and s differentiation and uses the 3M material as a common reflective polarizer. Moreover, while xe2x80x9cwide-anglexe2x80x9d performance is a widely recognized design goal, references to xe2x80x9cwide-anglexe2x80x9d are meaningless absent contrast limits and spectral leak reduction and teachings on how to achieve such a goal. The 3M product xe2x80x9cDBEFxe2x80x9d is a reflective polarizer with typical block direction leakages of 4 to 6 percent at normal incidence. At higher angles the leakage is somewhat reduced, but at 45 degrees the extinction is typically still a few percent. Contrast ratios when using DBEF typically will be limited to maximum values at or below 99:1 for white light. However, DBEF suffers from spectral leaks that reduce the contrast of certain color bands to as low as 25:1, depending on the nature of the illumination source and the exact DBEF sample. To obtain superior performance it is desirable that good screen uniformity and the absence of spectral leaks in the dark state accompany good average contrast in all relevant color bands.
The need remains for an optical imaging system that includes truly wide angle, fast optical components and that may allow viewing or display of high-contrast images.
The present invention describes an optical imaging system including and advantageously employing a wide-angle xe2x80x9cCartesianxe2x80x9d polarizer beam splitter (xe2x80x9cPBSxe2x80x9d). A Cartesian PBS is defined as a PBS in which the polarization of separate beams is referenced to invariant, generally orthogonal principal axes of the PBS film. In contrast with a MacNeille PBS, in a Cartesian PBS the polarization of the separate beams is substantially independent of the angle of incidence of the beams. The use of a Cartesian PBS film also allows the development of systems using curved PBS that provide higher light output and/or replace or augment other optical components.
A wide-angle PBS is defined as a PBS capable of receiving a cone of light rays with an angle of incidence up to 11xc2x0 or more, while maintaining acceptable system contrast. By recognizing and advantageously applying properties of wide-angle Cartesian polarizers, the present invention discloses a high-efficiency optical imaging system capable of functioning at f/#""s equal to or below f/2.5 while maintaining a contrast ratio of at least 100 to 1, or, more preferably, 150 to 1 in a projection system configuration.
An optical imaging system in accordance with the present invention includes a wide-angle Cartesian polarizing beam splitter, light valve illumination optics having an f/#xe2x89xa62.5, and at least one reflective light valve. The Cartesian polarizing beam splitter (PBS) has a structural orientation defining fixed polarization axes. A reflective Cartesian PBS substantially reflects those components of a beam of light which are polarized along one such fixed axis, called the Material Axis. Those components of a beam of light with polarization not along the Material Axis are substantially transmitted. The polarizing beam splitter therefore splits incident light into a first and a second substantially polarized beam having polarization states referenced to the fixed polarization axes and the polarizing beam splitter directs the first polarized beam onto the reflective light valve. In an exemplary embodiment, the Cartesian PBS includes 3M advanced film. In other exemplary embodiments, the PBS may include a wire grid polarizer, such as those described in Schnabel et al., xe2x80x9cStudy on Polarizing Visible Light by Subwavelength-Period Metal-Stripe Gratingsxe2x80x9d, Optical Engineering 38(2), pp. 220-226, February 1999, relevant portions of which are hereby included by reference. Other suitable Cartesian polarizers also may be employed.
The light valve illumination optics have an f/# of at most 2.5, a minimum cone angle of about 12 degrees and the system has a contrast ratio exceeding 100 to 1 using an ideal imager. In preferred embodiments, the contrast ratio exceeds 150 to 1 and the illumination optics have an f/# equal or less than 2.0. The illumination optics are those optics that condition (e.g., prepolarize, homogenize and filter) the light beam. The f/# is associated with the beam of light incident on the imager.
The light valve may be a polarization modulating light valve, including smectic or nematic liquid crystal light valves. The optical imaging system may further comprise a pre-polarizer that polarizes input light into pre-polarized light, the pre-polarized light comprising the incident light on the polarizing beam splitter. The optical imaging system also may include a color separation and recombination prism or mirrors and a plurality of reflective light valves. The prism receives the polarized light from the polarizing beam splitter, color separates the polarized light and directs polarized color beams to each light valve. The optical imaging system may include a suitable light source that supplies the incident light.
In alternative embodiments, the reflective light valve may reflect at least a portion of the first polarized beam back to the original polarizing beam splitter or to a second PBS.